This application is based on Japanese Patent Application No. 2000-268372 filed in Japan, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a positioning technique for using a signal transmitted from an artificial satellite to measure a position of an information terminal provided to a mobile station side system.
2. Related Art Statement
Japanese Patent Laid-Open No. 11-34996 describes a positioning technique using a non-geostationary general-purpose satellite that moves on a long elliptical orbit and can provide services without being affected by landforms and shadows by building arrangement within a specific service area (for example, the area of a certain country, such as the whole of Japan including isolated islands and the range of territorial waters) (hereinafter, referred to as a quasi-zenithal satellite), wherein the satellite is provided with a communication system, goes around on the elliptical orbit in a 24-hour cycle, and is used for the positioning in the range of orbit inclination of more than or equal to 37 degrees and less than or equal to 44 degrees and in the range of eccentricity of more than or equal to 0.24 and less than or equal to 0.35 (hereinafter, referred to as an xe2x80x9cHEO satellitexe2x80x9d).
A method of detecting a position by a navigation apparatus utilizing a non-geostationary general-purpose satellite is described in Japanese Unexamined Patent Laid-Open No. 10-48310. According to the detection method of 10-48310, a terrestrial station concerned transmits an RF signal to a user station through a forward link (satellite communication), and conversely, the user station replies to the terrestrial station through a return link (satellite communication). Then, based on the round-trip propagation time of this communication, the range between the terrestrial station and the user terminal (the sum of the distance between the terrestrial station and the non-geostationary general-purpose satellite and the distance between the geostationary general-purpose satellite and the user terminal) is calculated. Further, based on thus-calculated range and a known range between the geostationary general-purpose satellite and the terrestrial station, the user station performs positioning calculation of the range between the satellite and the user terminal. Further, this positioning calculation is performed based on the solutions by respective Doppler effects generated between the terrestrial station concerned and the satellite and between the satellite and the user station, and based on the above-calculated range.
Further, Japanese patent Laid-Open No. 8-331033 describes positioning utilizing elliptical orbit communication satellites. In particular, its paragraph 0081 describes calculation of distance between a communication satellite and a mobile station by obtaining a difference between a radio wave propagation time of a radio channel making a round trip between a satellite communication fixed station and a mobile station through the communication satellite and a radio wave propagation time of a radio channel making a round trip between the satellite communication fixed station and the communication satellite, and by multiplying the obtained difference by the radio wave propagation velocity. Further, the paragraph 0084 of the same document describes that received field strengths of radio channels between a satellite communication fixed station or a ground communication terrestrial station and a mobile station are obtained, and a radio channel having the largest received field strength among those radio channels is selected, and the selected channel is used to connect a communication line.
In the positioning method described in 11-34996, an HEO satellite, which is quasi-zenithal, is used for positioning. However, it does not describe what positioning method is favorable. In particular, it does not consider making Geometrical Dilution Of Precision (GDOP), which expresses the positioning precision, less than or equal to 10 (less than or equal to 11 or 9, when an error of 10% for the GDOP value 10 is included) at all.
Further, the positioning method described in 10-48310 utilizes general-purpose non-geostationary satellites. However, communication is performed bilaterally (forward link and return link) between a terrestrial station concerned and a user station through a satellite, and the terrestrial station concerned performs positioning calculation. Accordingly, the terrestrial station must perform transmissions and receptions four times, in order to perform positioning calculation. Further, in order that the user station itself can know its position, it must perform transmission and reception, further. In other words, reduction of positioning time including times for transmissions and receptions is not taken into consideration.
Considering the environment of the present communication system that the communication capacity is 2 MBPS or less while there are millions of users of information terminals having a positioning function such as car navigation terminals, it is difficult that the mentioned positioning method ensures the real time property of positioning in, for example, a car navigation system. Further, this conventional technique also does not consider how GDOP, which expresses a positioning precision, can be made not more than 10, at all.
Further, as described above, the positioning method of 8-331033 obtains a distance between a satellite communication fixed station and a communication satellite based on a radio wave propagation time of a radio channel that makes a round trip between the satellite communication fixed station and the communication satellite. Thus, it does not consider a delay error of radio wave propagation generated in the ionosphere when a radio channel makes a round trip between the satellite communication fixed station and the communication satellite, and an error of radio wave propagation time generated between a clock provided in the satellite communication fixed station and a clock provided in the communication satellite. In other words, it does not consider high precision positioning that suppresses effects of a radio channel making a round trip between the satellite communication fixed station and the communication satellite on radio wave propagation. In particular, it does not consider making GDOP not more than 10, at all.
Further, as described above, the conventional positioning methods do not considere making GDOP (Geometrical Dilution Of Precision) not more than 10, though it is necessary to make GDOP less than or equal to 10 as described in xe2x80x9cGPSxe2x80x9d, p. 135 (published by Japanese Association of Surveyors on Nov. 5, 1989). In other words, the conventional methods do not consider what operating conditions can make GDOP less than or equal to 10 when a quasi-zenithal satellite (in particular, an HEO satellite) is used for positioning.
Further, when there are a plurality of terrestrial station side systems, a terrestrial station through which connection with a telecommunication business is established is not selected in consideration of reduction of communication time and communication cost between the telecommunication business and the terrestrial station side system.
An object of the present invention is to provide an information terminal and positioning system having a positioning function that ensures a high positioning precision in a short time.
Another object of the present invention is to make CGOP, which is an index of positioning precision, less than or equal to 10, in particular, in positioning utilizing quasi-zenithal satellites (particularly, HEO satellites).
Further, another object of the present invention is to reduce a communication time and communication cost in providing Internet connection service through satellites.
The present invention provides a positioning system provided with:
(1) a terrestrial station side system comprising: a terrestrial station side satellite communication antenna for transmitting a signal to a satellite; and a terrestrial station side communication apparatus for transmitting a signal to the terrestrial station side satellite communication antenna;
(2) the satellite having a satellite side satellite communication antenna for transmission and reception to and from the ground; and
(3) a mobile station side system comprising: a mobile station side satellite communication antenna for receiving the signal from the satellite; and an information terminal for measuring a position where the mobile station side system exists, based on the signal received through the mobile station side satellite communication antenna.
Further, the present invention provides: a positioning method used for the positioning system of the present invention; an information terminal (including a portable terminal) that can perform positioning using the mentioned method; a computer readable storage medium that stores a computer program for realizing the mentioned positioning method; and a computer program product having computer readable program code means for realizing the mentioned positioning method.
The positioning method of the present invention comprises steps of:
extracting a distance between a position where a terrestrial station side system exists and a satellite, from a signal transmitted from the satellite;
extracting a transmission time from the signal transmitted from the satellite;
obtaining a sum of the distance (a) between the terrestrial station side system and the satellite and a distance (b) between the satellite and a mobile station side system;
obtaining the distance (b) between the mobile station side system and the satellite by subtracting the distance (a) between the terrestrial station side system and the satellite from the sum (a+b); and
obtaining a position where the mobile station side system exists, based on the obtained distance (b) between the terrestrial station side system and the satellite.